High intensity discharge (HID) lamps typically require the application of a starting voltage or ignition voltage that is substantially higher than the operating voltage of the lamp. This starting voltage must provide a sufficiently higher electric field, such that, in the presence of an avalanche-initiating electron, breakdown will occur. It is well known to those skilled in the art that igniting HID lamps can be difficult, especially in lamps using high buffer gas pressures, in mercury-free lamps or in re-start situations after a lamp has recently been extinguished.
Many attempts have been made to improve the starting of HID lamps. For example, some ignition aids improve the starting performance by assuring the presence of an avalanche-initiating electron. Specifically, the use of UV enhancers and Krypton-85 containing buffer gases is well known. Other methods and devices are intended to enhance the local electric field in the region between the electrodes (or in the discharge volume for electrodeless lamps). Another method of aiding the initiation of a discharge involves increasing the electric field at a given externally applied voltage. It is to the latter category that the instant invention pertains.
Typically, such field enhancement is accomplished by the addition of an electrically conductive member such as a wire or metallized stripe, which reduces the effective arc gap between the electrodes, thus leading to a lower breakdown voltage. The conductor can be floating, as in the case of high pressure sodium lamps, (see, for example, U.S. Pat. No. 6,661,171), or the conductor can be electrically coupled to one of the electrodes. Connection to one of the electrodes introduces an undesirable influence on sodium migration in the case of metal halide or sodium lamps, so a bimetal switch typically is employed to disconnect the starting aid from the electrode as the lamp heats up.
In electrodeless lamps, it has been suggested to embed a conductor into the quartz envelope to provide field enhancement (see, for example, U.S. Re32,626). The deposition of a matrix coating of conductive and/or semi-conductive fibers has also been suggested to facilitate starting. The deposition can be internal or external and, if internal, it is suggested that the fibers be coated with a sol gel-deposited silica coating to protect the fibers from the plasma environment (see U.S. Pat. No. 6,628,079).
While the above methods have had success in the various large lamps currently in use, it would be an advance in the art if a reliable starting aid could be supplied for small, ceramic HID lamps.